Method for in-situ markers for thermal mechanical structural health monitoring

ABSTRACT

A method of monitoring the residual stress in surface and near surface regions of a component includes identifying predetermined locations on the surface of a component that are expected to experience high stress during normal operating conditions of the component. Marker particles are introduced into the component during additive manufacture of the component at the predetermined locations. Then, the residual stress of the component is measured at a location corresponding with the marker material using x-ray techniques.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. application Ser. No. 15/112,127filed Jul. 15, 2016 for “METHOD FOR IN-SITU MARKERS FOR THERMALMECHANICAL STRUCTURAL HEALTH MONITORING”, which in turn claims thebenefit of PCT International Application No. PCT/US2014/050296 filedAug. 8, 2014 for “METHOD FOR IN-SITU MARKERS FOR THERMAL MECHANICALSTRUCTURAL HEALTH MONITORING”, which in turn claims the benefit of U.S.Provisional Application No. 61/868,297 filed Aug. 21, 2013 for “METHODFOR IN-SITU MARKERS FOR THERMAL MECHANICAL STRUCTURAL HEALTH MONITORING”by L. Dautova, W. V. Twelves, Jr., J. Ott, E. Butcher, G. A.Schirtzinger and R. J. Hebert.

BACKGROUND

This invention relates generally to the field of additive manufacturing.In particular, the invention relates to an additive manufacturingprocess enabling the measurement of residual stresses at specificlocations in components.

Additive manufacturing is a process by which parts can be made in alayer-by-layer fashion by machines that create each layer according toan exact three dimensional (3D) computer model of the part. In powderbed additive manufacturing, a layer of powder is spread on a platformand selective areas are joined by sintering or melting by a directedenergy beam. The platform is indexed down, another layer of powder isapplied, and selected areas are again joined. The process is repeatedfor up to thousands of times until a finished 3D part is produced. Indirect deposit additive manufacturing technology, small amounts ofmolten or semi-solid material are applied to a platform according to a3D model of a part by extrusion, injection or wire feed and energized byan energy beam to bond the material to form a part. Common additivemanufacturing processes include selective laser sintering, direct lasermelting direct metal deposition, and electron beam melting.

Once the component is manufactured, the component is incorporated into asystem to be used for a specific function. An example is a gas turbineengine. During operation, the component is exposed to thermal andmechanical environments that stress the component. The stresses andresulting strain experienced by the component cause residual stressesand possible structural failures or cracks in the component.

Several non-destructive techniques exist to detect crack growth orresidual stresses in components. Current non-destructive techniquesexpose the component to external probes such as electromagnetic fields,dyes, or ultrasonic waves. It is difficult to obtain localizedinformation at pre-determined locations in a component with the currenttechnologies, for example at regions of increased service stresses.Current technologies mostly detect flaws after they have formed, and arefar less sensitive to the stage leading up to the formation of flaws,for example, internal cracks.

SUMMARY

A method of monitoring the residual stress of a component of a basealloy formed by additive manufacturing includes identifyingpre-determined locations on the component that experience high stressduring normal operating conditions of the component. Marker particlesare introduced into surface and near surface regions of the componentduring additive manufacture of the component at the pre-determinedlocations. The residual stress of the component is measured at themarker particle locations.

A component formed by additive manufacturing and further subjected tostress during operation contains marker materials inserted in thesurface and near surface regions of the component at variouspredetermined locations over the surface of the component. The markersallow residual stress measurements to be made on the component at thesite of each marker material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram representing the method of monitoring residualstress in a component.

FIG. 2 is a schematic diagram of a direct metal deposition process.

FIG. 3 is a perspective view of a turbine blade.

DETAILED DESCRIPTION

FIG. 1 is a flow diagram representing the method of monitoring residualstress. Method 10 involves, first, the identification of high stressregions caused by normal use conditions of a component (Step 12). In thenext step, an additive manufacturing process is initiated (Step 14).Marker particles are introduced at pre-determined high stress locationsof the component during manufacture (Step 16). The additivemanufacturing process is then completed (Step 18), and the component isput into service under normal use conditions (Step 20). The component isremoved from service when necessary (Step 22). X-ray diffractionmeasurements are made in the regions where the marker particles arelocated (Step 24) to determine interplanar spacing of the markerparticles in order to determine local internal elastic strain andinternal residual stress. (Step 26).

X-ray diffraction techniques to measure residual stress in a metalcomponent are well-known in the art and rely on the fact that aninternal elastic stress will change the interplanar spacing of acrystalline solid under stress from the interplanar spacing of the samematerial in a stress-free state. The interplanar spacing is determinedfrom the well-known Bragg's law

nλ=2d sin θ

where λ is the incident x-ray wavelength, d is the interplanar spacing,θ is the diffraction angle of a diffraction peak and n is an integer. Ifd₁ is the interplanar spacing of a stressed metal in a certaincrystallographic direction and d₀ is the spacing of the same metal inthe same direction in a stress free state, the residual strain in thatdirection c is:

$ɛ = \frac{d_{1} - d_{0}}{d_{0}}$

Residual stress in an elastically isotropic material can be determinedfrom the strain by multiplying the strain with an appropriate termcontaining the elastic modulus and Poisson's ratio. An example referencediscussing x-ray measurements of residual stress is “Determination ofResidual Stresses by X-ray Diffraction—issue 2” by Fitzpatrick et al.,National Physical Laboratory of the UK (available at www.npl/co/uk),which is incorporated herein in its entirety.

The marker particles inserted into the additive manufactured componentare chosen as to not interact with the component material by alloying,by the formation of second phases or by other forms of solution orinteraction. The interplanar spacing change of the markers can then beused as a measure of internal stress of a component containing themarkers in the vicinity of the markers. The interplanar spacings of themarker material in the as-built condition are taken to be the stressfree reference values.

The marker particles are inserted into the surface and near surfaceregions of the component for the measurement of residual stress byx-rays. The penetration of x-rays into a metal component is typically onthe order of a few microns.

An example marker material for use with titanium alloy turbinecomponents such as Ti-6Al-4V is cerium. Cerium is nearly insoluble intitanium, there are no intermetallic compounds in the Ti—Ce binarysystem, and cerium, due to its large atomic mass, produces a relativelystrong x-ray signal. Dysprosium and samarium are other candidates.

An additive manufacturing process suitable for use with the presentmethod is direct metal deposition (DMD). A schematic of a direct metaldeposition process is shown in FIG. 2. DMD process 30 includes base 32,workpiece 34, deposition unit 36 and sensors 46 and 48. Base 32 iscapable of three axis computer controlled positioning as schematicallyindicated by arrows A. Deposition unit 36 contains channels 40 and 42that may carry deposition powders and inert gas to the deposition site.Deposition unit 36 further contains a laser energy source (not shown)and associated optics 38. Deposition unit 36 is capable of five axiscomputer controlled positioning during a build. Output from sensors 44and 46 is used to control the build of workpiece 34. Workpiece 34 isformed by laser 38 melting small region 48 on workpiece 34 into whichpowders are introduced through channels 42 and 44. The build is apoint-by-point process according to a CAD model of workpiece 34 modelstored in memory of the control system of device 30.

In the present invention, when marker regions are required, markerparticles replace the normal build particles being deposited in meltpool 48. The size of the marker regions may be from 0.1 microns to overa millimeter depending on the requirements.

A perspective view of exemplary turbine blade 50 is shown in FIG. 3.Turbine blade 50 comprises root 52, platform 54, airfoil 56 with coolingpassages 58 and tip 60. High stress regions on blade 50 during serviceare predominantly in transition regions such as between airfoil 56 andplatform 54 and in curved regions in root 52 all indicated by arrows T.It is these regions where markers of the present invention are placedduring an additive manufacturing build such as that shown in FIG. 2 tomonitor the residual stress generated during service. The markermaterials, to be useful, should exhibit at least the followingcharacteristics. They should not alloy with the base alloy during theadditive manufacturing process. They should not form intermetallicphases with the base alloy. They should preferably have a high atomicnumber for strong X-ray signature. In addition, the diffraction peaks ofthe marker materials preferably should not overlap the diffraction peaksof the base alloy.

Discussion of Possible Embodiments

The following are non-exclusive descriptions of possible embodiments ofthe present invention.

A method of monitoring residual stress of a component of a base alloyformed by additive manufacturing may include: identifying a high stresslocation of a component that experiences high stress during normaloperating conditions of the component; introducing during additivemanufacturing marker particles in surface and near surface regions tocreate a marker associated with the identified high stress location ofthe component; and measuring a residual stress of the component at themarker.

The method of the preceding paragraph can optionally include,additionally and/or alternatively any, one or more of the followingfeatures, configurations and/or additional components:

Measuring the residual stress at the marker with x-ray diffraction;

The x-ray diffraction may be used to measure an interplanar spacing ofthe marker in at least one of the surface and near surface predeterminedlocations;

A local strain in the marker may be determined from the measuredinterplanar spacing;

The x-ray diffraction measurement may be performed with an x-raydiffractometer;

The x-ray diffraction may be performed with x-ray beams of about 1 mm to2 mm in diameter focused on a surface of the component;

The additive manufacturing may include direct metal deposition, directlaser melting or direct laser deposition;

The marker may be insoluble in the base alloy;

The base alloy may include a titanium alloy and the marker may becerium.

A component of a base alloy formed by additive manufacturing that issubjected to stress during operation may include a marker of a markermaterial different from the base alloy inserted in surface and nearsurface regions of the component at a predetermined location to allowresidual stress measurements to be made on the component at the marker.

The component of the preceding paragraph can optionally includeadditionally and/or alternatively any, one or more of the followingfeatures, configurations and/or additional components:

The predetermined location may be a region expected to undergo stressduring normal operating conditions of the component;

The residual stress measurements may be x-ray diffraction measurements;

The x-ray diffraction measurements may be used to determine residualstrain in the marker material by measuring lattice interplanar spacingof the marker material;

The x-ray diffraction measurements may be performed with an x-raydiffractometer;

The x-ray diffraction measurements may use beam sizes of about 1 mm to 2mm;

The additive manufacturing may include direct metal deposition, directlaser melting or direct laser deposition;

The marker material may be insoluble in the base alloy and may not forma second phase with the base alloy and otherwise may not react with thebase alloy;

The base alloy may be a titanium alloy and the marker material may becerium.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

1. A component of a base alloy, formed by additive manufacturing, thatis subjected to stress during operation, the component comprising amarker of a marker material different than the base alloy inserted insurface and near surface regions of the component at a predeterminedlocation to allow residual stress measurements to be made on thecomponent at the marker.
 2. The component of claim 1, wherein thepre-determined location comprises a region expected to undergo stressduring normal operating conditions of the component.
 3. The component ofclaim 1 wherein the residual stress measurements are x-ray diffractionmeasurements.
 4. The component of claim 2 wherein the x-ray diffractionmeasurements are used to determine residual strain in the markermaterial by measuring lattice interplanar spacing of the markermaterial.
 5. The component of claim 3 wherein the x-ray diffractionmeasurements are performed with an x-ray diffractometer.
 6. Thecomponent of claim 3 wherein the X-ray diffraction measurements use beamsizes of about 1 mm to 2 mm.
 7. The component of claim 1 whereinadditive manufacturing comprises direct metal deposition, direct lasermelting or direct laser deposition.
 8. The component of claim 1 whereinthe marker material is insoluble in the base alloy, does not form asecond phase with the base alloy and otherwise does not react with thebase alloy.
 9. The component of claim 1 wherein the base alloy comprisestitanium alloy and the marker material comprises cerium.